This invention relates to an atmospheric polymer electrolyte membrane (PEM) fuel cell having stochiometry controlled by a schedule of blower power as a function of load current.
The amount of air consumed in a fuel cell by the fuel cell process is called the xe2x80x9cstochiometricxe2x80x9d amount. The ratio of (a) the total amount of air supplied to the oxidant flow field to (b) the stochiometric amount of air is typically (and herein) referred to as xe2x80x9cstochiometryxe2x80x9d. Sometimes stochiometry is referred to as a percentage: thus, a stochiometry of 333 implies 333%, and is the same as a stochiometry of 3.3 (as used herein). The phase xe2x80x9cair utilizationxe2x80x9d refers to the reciprocal of stochiometry, and is the percent of total air which is consumed: utilization of 30 (implying 30%) equals stochiometry of 3.3 or 333 (implying 333%).
Most conventional fuel cells known to the prior art are operated with a substantially constant stochiometry of between 2 and 3.5, typically. Such a choice strikes a balance between the additional oxidant required at high load currents and the need to limit the wasted parasitic power which results from excessive compression of oxidant. It has been suggested in U.S. Pat. No. 5,366,821 that a PEM fuel cell operating at one or two atmospheres can adjust air flow either (1) to keep the output voltage constant for any current load, or (2) to provide optimal operation in which the parasitic power (mostly utilized to operate a compressor) is minimized, or (3) to maintain a fixed oxygen utilization ratio. In that patent, the compressor provides compressed air to a storage tank which has a setpoint pressure, and the compressor is utilized to maintain the setpoint pressure in the air storage tank. Flow regulation is accomplished by calculating a desired flow rate, monitoring the air flow input to, and current output of the fuel cell, and regulating the air flow through the fuel cell by means of flow control valves. While describing achievement of the foregoing objectives separately, the achievement of one objective precludes achievement of either of the other objectives. The choices include: controlling flow and stochiometry for a constant voltage at all load currents, at a penalty of high parasitic power; controlling flow for minimized parasitic power at a penalty of wide variations in output voltage and power as a function of load current; and controlling flow for a constant stochiometry at a penalty of variable voltage and power as a function of load current and increased parasitic power at both higher load current and lower load current. The parasitic power in any PEM fuel cell employing a compressor will be prohibitive for use in vehicles.
Objects of the invention include improved PEM fuel cell operation at near atmospheric pressure; PEM fuel cells operating near atmospheric pressure having output voltage which decreases moderately as a function of load current; and PEM fuel cells operating near atmospheric pressure having improved characteristics rendering them more suitable for use in vehicles.
This invention is predicated on the discovery that a moderate increase in air stochiometry as a function of load current will improve the operating characteristics of PEM fuel cells operating at substantially atmospheric pressure.
According to the present invention, the mass flow rate of air in PEM fuel cells operating at substantially atmospheric pressure is increased beyond stochiometric amounts so as to control stochiometry as a function of load current, in response to a schedule of pump or blower power as a function of load current. In accordance further with the invention, the stochiometry of a PEM fuel cell operating near atmospheric pressure is increased as a function of load current by increasing the speed of an oxidant pump or blower at the inlet or outlet of an oxidant flow field. In further accord with the invention, the stochiometry is increased at a rate of between 1.7 per amp/cm2 and 2.5 per amp/cm2, for current densities above a threshold. The pump or blower may be controlled by variable input voltage, by duty cycle switching of input voltage, or in any other known fashion.
The present invention recognizes that neither the parasitic power, the voltage, nor any other parameter of a fuel cell can be controlled to its optimum while at the same time controlling another of such parameters to a corresponding optimum. Instead, the invention recognizes that there are advantages to variable stochiometry which can be achieved for a number of parameters at one time, if the stochiometry is altered in an appropriate fashion. For instance, even though voltage cannot remain constant, limiting the variations in voltage as a function of load current simplifies the power conditioning equipment which is used as a part of every fuel cell power plant, thus reducing the size, the weight and the cost thereof. This renders the fuel cell, with its power conditioning equipment, more suitable for certain uses, such as in vehicles. Furthermore, adjusting the stochiometry in accordance with the present invention, in contrast with utilizing a fixed stochiometry which is conventional in the prior art, improves the water self-sufficiency of the fuel cell process.